1. Field of the Invention
The present invention relates to a color image sensor and, more particularly, to a contact type color image sensor wherein photocells having a plurality of color filters are aligned in line.
2. Related Background Art
Typical conventional original reading systems using color image sensors include systems for reducing the size of an image and reading a reduced image and systems for focusing and reading an image of an equal size. The latter system has been popular in recent years since adjustment is simple and the required amount of light is small. In the original reading system for focusing and reading an image of equal size, however, an elongated sensor is required.
Typical conventional color original reading systems include light source switching type systems and tricolor separation type systems using filters. The latter system is further classified into a tricolor sequential reading system for sequentially reading three color components by a single line sensor according to filter switching, a system for simultaneously reading three color components by using three line sensors, and a system for simultaneously reading three color components by using a single color line sensor chip having a tricolor stripe filter thereon.
A sensor chip with equal size type tricolor filters has recently received a great deal of attention since it can read a color image at high speed. However, in order to cause this sensor to read an image at the same reading density as that of a black-and-white sensor, the tricolor filter sensor chip must have a reading density three times that of the black-and-white sensor.
A CCD (Charge-Coupled Device) sensor is used as a typical conventional contact type color line sensor, as shown in FIG. 1. Photocells 10 each consisting of a red (R) filter, a green (G) filter, and a blue (B) filter are aligned in a line. Sensor parts of a sensor chip 100 include red filters, 1A, 2A, 3A, and 4A, green filters 1B, 2B, 3B, and 4B, and blue filters 1C, 2C, 3C, and 4C.
The sensor parts are arranged in the order of 1A, 1B, 1C, 2A, 2B, 2C, ..., and a set of R, G, and B sensor parts, e.g., 1A, 1B, and 1C, constitute a photocell 10 serving as one pel, thereby obtaining a predetermined resolution. For example, in order to read an image at a rate of 10 pels/mm for each color component, the width of one pel, i.e., a pitch p of pels is 1/16 mm. The width b of each sensor part 1A, 1B, 1C in the photocell 10 is narrower than p/3. For example, if p=62.5.mu.m, then the width b of each sensor part must be 20.8 .mu.m or less. If a light-shielding width a of about 5 .mu.m between the sensor parts is taken into consideration, the width b of each sensor part must be about 15.8 .mu.m. It should be noted that the sensor parts 1A, 1B, 1C, . . . are formed on the sensor chip 100.
This conventional sensor chip with equal size type tricolor filters requires a high-density elongated sensor, as described above. If photoelectric transducer elements are formed on a silicon wafer in order to use a CCD or MOS image sensor as the elongated sensor, the chip size of the image sensor is limited by the silicon wafer size. For example, if a 4" wafer is used, the chip size of the image sensor is 100 mm or less.
In order to constitute a line sensor having an A4 (210 mm) width, a plurality of image sensor chips must be arranged linearly or in a staggered manner.
FIG. 2 shows an example of a line sensor 110 obtained by arranging CCD sensor chips 100 (FIG. 1) in a staggered manner within the A4 width. The length of each CCD sensor chip varies according to the silicon wafer size. However, each of CCD sensor chips 100A to 100E has a length falling within the range of 50 to 60 mm. In order to arrange the CCD sensor chips to cover the A4 width (210 mm), at least four or five chips 100 are required. Since the light-shielding width a is very small, the adjacent chips 100 cannot be aligned in line with such a small gap. As a result, the staggered arrangement is inevitably employed, as shown in FIG. 2.
With the arrangement in FIG. 2, a distance k between the upper and lower rows must be corrected in the read mode. For this purpose, an external correction line memory or buffer memory must be arranged, and at the same time processing is required to link the data read by the adjacent line sensors.
On the other hand, the linear arrangement of the line sensor does not require a buffer memory and data link processing, unlike the staggered arrangement. However, in order to eliminate a read disable area, photoelectric transducer elements such as photodiodes must be arranged to the very end of the chip. The chip must then be cut or scribed with higher precision, and the resultant chips must be aligned in line on a substrate with higher precision.
In particular, a color sensor requires a tricolor separation filter unit so that the reading density of the photoelectric transducer elements in the main scanning direction (i.e., the line direction) is three times that of a black-and-white sensor. For example, as described above, if the reading density is 16 pels/ mm, the element density of the color sensor is 48 elements/mm, and the element pitch is about 20 .mu.m. It is very difficult to form the photodiodes within this element pitch and to precisely cut the wafer without damaging the photodiodes.
Since the spectral sensitivity of the blue (B) filter is normally poor. the level of the blue signal as a sensor output is low and an S/N ratio is undesirably low.